Capacitive pressure measuring cell with a membrane bed

ABSTRACT

The invention relates to a pressure measuring cell comprising a base body, a membrane that is connected to the base body, thus forming a measuring chamber. During operation, the membrane is subjected to a deflection which is dependent on a pressure that is to be measured. The cell also comprises a membrane bed, formed by a surface of the base body that faces the membrane, at least one electrode being mounted on said bed. Said electrode, together with a counter-electrode that is mounted on the membrane, forms a capacitor, whose capacitance represents a measurement for the deflection of the membrane. The measuring cell is characterized in that the electrode is electrically connected through the base body and that the measuring chamber has a smooth surface in the contact zone, said surface having a contact pin that is guided in a bore through the base body. The electrode is electrically connected by means of said pin for measuring the capacitance and the pin is soldered into the bore on the membrane side, using a solder. The contact pin, the solder and the membrane bed form a smooth surface.

FIELD OF THE INVENTION

The present invention relates to a capacitive pressure measuring cell.

BACKGROUND OF THE INVENTION

In pressure measuring technology, absolute-pressure, relative-pressureand differential-pressure measuring cells are used. In absolute-pressuremeasuring cells, a pressure to be measured is detected in absolute form,that is, as a pressure difference compared to a vacuum. With arelative-pressure measuring cell, a pressure to be measured is picked upin the form of a pressure difference compared to a reference pressure,such as a pressure that prevails where the sensor is located. In mostapplications, at the location of use this is the atmospheric pressure.Accordingly in absolute-pressure measuring cells, a pressure to bemeasured is detected with reference to a fixed reference pressure, thatis, the vacuum pressure, and in relative-pressure measuring cells, apressure to be measured is detected with reference to a variablereference pressure, such as the ambient pressure. Adifferential-pressure measuring cell detects a difference between afirst and a second pressure applied to the measuring cell.

There are pressure measuring cells on the market, having: a base body; amembrane, connected to the base body, forming a measuring chamber, whichin operation executes a deflection that is dependent on a pressure to bemeasured; and an electrode, disposed in the measuring chamber on a sideof the base body toward the membrane, which electrode together with acounter-electrode applied to the membrane forms a capacitor, whosecapacitance is a measure for the deflection of the membrane.

In such pressure measuring cells, an electrical connection of theelectrode can be accomplished either through an interstice between themembrane and the base body, or through the base body. In the firstinstance, a joining material, by which the membrane and the base bodyare connected to one another, must be an electrical insulator.Contacting through the base body is to be preferred, since it leaves theconnection between the base body and the membrane unimpaired and thusleaves it tight and mechanically stable, and since it involves nolimitation in the selection of the joining material.

In conventional contacting through the base body, a metal contact pin isinserted into a bore that penetrates the base body and is compressed onthe end, for instance with an arbor. As a result, the contact pin ismechanically fixed, and an electrical contact point with the electrodeof the base body is created.

In most applications, this method furnishes very good results and can beperformed quickly and economically. However, applications exist in whichthis method of compressing is disadvantageous.

The compressing does not achieve much tightness. Vacuum tightness of thekind required in absolute-pressure measuring cells is unattainable.Consequently, especially with absolute-pressure measuring cells, aseparate sealing of the through-connection is necessary.

Because of the compressing, an inside face of the measuring chamber inthe region of the bore and the metal pin is uneven and has recesses,such as gaps or indentations.

These irregularities in the inside face cause problems whenever theinside face is to be used as a membrane bed, to which the membraneconforms in the event of an overload. Irregularities in the membrane bedin the event of an overload can lead to permanent changes in themembrane that can later cause grave errors of measurement or evencomplete failure of the pressure measuring cell.

Geometric irregularities, in particular gaps, in the region of theelectrical contact point can lead under some circumstances to elevatedtransition resistances, with attendant disadvantages in picking up themeasured value.

Further problem arise if the measuring chamber is to be filled with apressure mediator fluid. In fluid-filled pressure measuring cells,preferably only a very slight volume of fluid is used, since thermalexpansion of the fluid from temperature must be kept as slight aspossible. It is correspondingly important for the measurement precisionthat the volume in which the liquid is placed be as constant aspossible. Dents, gaps or other forms of recesses that a fluid volume ofunknown size can penetrate over the course of time must absolutely beavoided. In differential-pressure measuring cells, it is especiallyimportant not only that the volume be as constant as possible but alsothat as much as possible the same quantity of fluid is present in bothhalves of the differential-pressure measuring cell. Unequal fluidquantities result in unequal temperature courses, which have a directeffect on the measurement precision.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a pressure measuringcell in which an electrode disposed on the base body in the measuringchamber is connected electrically through the base body, and in whichthe measuring chamber in the region of the contacting has a smoothsurface and a vacuum-tight and pressureproof connection.

To that end, the present invention comprises a pressure measuring cell,having: a base body; a membrane, connected to the base body, forming ameasuring chamber, which in operation executes a deflection that isdependent on a pressure to be measured; a membrane bed, formed by asurface, oriented toward the membrane, of the base body, onto which bedat least one electrode is applied, which together with acounter-electrode applied to the membrane forms a capacitor, whosecapacitance is a measure for the deflection of the membrane; and acontact pin, passed through a bore through the base body, by way ofwhich pin the electrode is electrically connected for measuring thecapacitance, and which toward the membrane bed is soldered into the borewith a solder, with the contact pin, the solder, and the membrane bedforming a smooth surface.

In one embodiment, the base body and/or the membrane is of ceramic or amonocrystal.

In a further embodiment, the contact pin comprises tantalum, and thesolder is an active hard solder, in particular a silver-copper solder.

In a further embodiment, in operation a pressure to be measured acts onthe membrane, and a very small pressure near 0 mbar or a referencepressure that is delivered through the base body prevails in themeasuring chamber.

In a further embodiment, the membrane is disposed between the base bodyand a further base body; the membrane and the further base body form afurther measuring chamber, and in operation, a first pressure prevailsin the measuring chamber; a second pressure prevails in the furthermeasuring chamber. The deflection of the membrane depends on a pressureto be measured, which is equivalent to the difference between the firstand the second pressure.

In a further feature of this latter embodiment, the base body has acontinuous bore leading into the measuring chamber, into which bore apressure tube is introduced, by way of which in operation the firstpressure is delivered to the measuring chamber; and the further basebody has a continuous bore leading into the measuring chamber, intowhich bore a pressure tube is introduced, by way of which in operationthe second pressure is delivered to the measuring chamber.

Furthermore, the present invention comprises a method for producing apressure measuring cell, in which: the contact pin is inserted into thebore; between the base body and the contact pin, a solder is introducedtoward the membrane bed; the contact pin is soldered in; the membranebed is polished; the electrode is applied to the smooth membrane bed;and the base body and the membrane are connected solidly together byjoining.

In a feature of the method, the membrane is connected to the base bodyand to the further base body by means of a joint; and the first pressuretube is soldered into the base body and the second pressure tube issoldered into the further base body.

The invention and further advantages will now be described in furtherdetail in conjunction with the drawing figures, which show threeexemplary embodiments. In the drawings, identical elements areidentified by the same reference numerals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a section through a pressure measuring cell of theinvention that is constructed as a capacitive absolute-pressuremeasuring cell;

FIG. 2 shows a section through a pressure measuring cell of theinvention that is constructed as a capacitive relative-pressuremeasuring cell; and

FIG. 3 shows a section through a pressure measuring cell of theinvention that is constructed as a capacitive differential-pressuremeasuring cell.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a section through a pressure measuring cell of theinvention. It has a cylindrical base body 1 and a circular-disk-shapedmembrane 5 that is connected to the base body 1, formed a measuringchamber 3.

The membrane 5 and base body 1 comprise an insulator, preferably ofceramic or a monocrystal, such as sapphire. They are connected to oneanother by a joint 6, for instance by means of an active hard solder,such as a zirconium-iron-titanium-beryllium solder, each on a respectiveouter annular edge.

In operation, the membrane 5 experiences a deflection that is dependenton a pressure p to be measured. To that end, the pressure p acts on themembrane 5, for instance on its outside. This is representedsymbolically by an arrow in FIG. 1.

A surface of the base body 1 oriented toward the membrane forms amembrane bed. An electrode 7 is applied to the membrane bed and togetherwith a counter-electrode 9 applied to the membrane 5 forms a capacitor,whose capacitance is a measure for the deflection of the membrane 5.

The counter-electrode 9, on its outer edge, electrically conductivelyadjoins the joint 6 and is preferably connected to ground or to a fixedreference potential via the electrically conductive joint 6.

The base body 1 has a continuous bore 11, through which a contact pin 13is passed. The contact pin 13 comprises an electrically conductivematerial and is soldered into the bore 11 toward the membrane bed with asolder 14. For the contact pin 13, tantalum for instance is especiallysuitable, since it is a ductile material with a high melting point. Itis resistant to nearly all acids and alkalis and is especiallycorrosion-resistant. If an aluminum oxide ceramic is used for themembrane 5 and the base body 1, tantalum offers the further advantagethat it has a coefficient of thermal expansion that is very similar tothat of the aluminum oxide ceramic. As the solder, an active hardsolder, in particular a silver-copper solder, is especially highlysuitable. With it, a hermetically sealed closure between the contact pin13 and the bore 11 can be established.

According to the invention, the contact pin 13, solder 14, and membranebed form a smooth surface. A clean, smooth surface, for instance apolished surface, offers the advantage that it forms a homogeneouscapacitance. Such a capacitance makes a flat connection with theelectrode 7 applied later, by way of which connection a highlyconductive electrical connection is achieved.

A further advantage is that because of the smooth surface at this point,a membrane bed that is smooth overall is created, to which the membrane5 can conform in the event of an overload, without suffering damage. Themembrane 5 is accordingly securely intercepted even at high overloads,for instance of 40,000 kPa (400 bar), and once the overload has faded,the measuring cell continues to function properly to its specifications.

The electrode 7 for measuring the capacitance is connected electricallyvia the contact pin 13. To that end, a first end of the contact pin 13is connected electrically conductively to the electrode 7. A remainingsecond end protrudes out of the base body 1 and is carried, in theexemplary embodiment shown, to an electronic circuit 15 disposed on thebase body 1. The electronic circuit 15 converts the changes incapacitance of the capacitor into an electrical output signal, such as acorrespondingly varying electrical voltage. The output signal isavailable for further processing and/or evaluation via connection lines17.

To achieve greater tightness and improved mechanical stability, thecontact pin can also be connected on the side remote from the membranebed to the bore 11 by a solder 19. This additionally offers theadvantage that mechanical forces cannot be transferred to the end of thecontact pin 13 toward the membrane bed and thus to the electricalcontact of the electrode 7. Forces exerted from outside are interceptedon the side remote from the membrane bed by the fastening to the solder19.

In the event that the materials adjoining one another in the region ofthe contacting have different coefficients of thermal expansion, andthat the pressure measuring cell is exposed under some circumstances toconsiderable temperature changes, it may be more favorable to solder thecontact pin 13 in place on only one end, or to fill the bore 11 over itsentire length with solder. This latter variant offers the advantage thattemperature-caused stresses are distributed over the full length.

It is understood that the invention is not limited to pressure measuringcells with a single electrode and to its connection according to theinvention via a soldered-in contact pin. It is understood that aplurality of electrodes may be provided, which are electricallyconnected in the manner described for the exemplary embodiments shown.

In FIG. 1, an absolute-pressure measuring cell is shown. The measuringchamber 3 is evacuated, so that in the interior of the measuringchamber, a very slight pressure near 0 mbar prevails, and the pressure pto be measured is detected with reference to the vacuum pressure in theinterior of the measuring chamber 3.

A reference-pressure measuring cell can be constructed in an entirelyanalogous way. An exemplary embodiment of this is shown in FIG. 2. It isdistinguished from the absolute-pressure measuring cell shown in FIG. 1solely in the fact that the measuring chamber 3 is not evacuated.Instead, a reference pressure p_(R) supplied through the base body 1prevails in the measuring chamber 3. The reference pressure p_(R) is forinstance a pressure that prevails in the environment surrounding themeasuring cell. For instance, as shown in FIG. 2, it can be introducedinto the measuring chamber 3 through a bore 21 that penetrates the basebody 1.

The pressure measuring cells shown in FIGS. 1 and 2 are produced in thatthe contact pin 13 is first inserted on the side toward the membrane bedflush at the front into the bore 11.

The solder 14, 19 is introduced between the base body 1 and the contactpin 13 on the side toward the membrane bed, and optionally also on aside facing the membrane bed. Next, the contact pin 13 is soldered inplace on one end or both ends in a furnace in a vacuum, or in aprotective gas atmosphere.

In the next work step, the membrane bed is polished, until it has asmooth surface that in particular, because of the solder 14, isgap-free.

The electrode 7 is applied, for instance by sputtering or vapordeposition, to this smooth and in particular gap-free surface. Theelectrode 7 preferably likewise comprises tantalum. However, othermetals can also be used.

Sputtered or vapor-deposited electrodes can be produced very preciselywith slight thicknesses, for instance of 0.1 μm. The smooth surface thatis attainable by soldering the contact pin 13 in place is especiallyadvantageous at these slight layer thicknesses, since even at slightlayer thicknesses, it offers a good electrical contact face.

By the same method, the membrane 5 is also provided with thecounter-electrode 9, and in a final work step, the joint 6 between themembrane 5 and the base body 1 is made. To that end, the joiningmaterial, such as the aforementioned zirconium-iron-titanium-berylliumactive hard solder, is applied to an annular-disk-shaped surface on theedge of the base body 1, and the membrane 5 is placed on that. The basebody 1 and membrane 5 are solidly connected to one another by joining ina furnace in a vacuum or in a protective gas atmosphere.

In FIG. 3, a further exemplary embodiment of a pressure measuring cellof the invention is shown. This one is a differential-pressure measuringcell. It has a base body 23 and a further base body 25. A membrane 27 isdisposed between the base body 23 and the further base body 25. Themembrane 27 is connected to the base body 23, forming a measuringchamber 29, and to the further base body 25, forming a further measuringchamber 31.

The membrane 27, base body 23, and further base body 25 comprise aninsulator, preferably of ceramic or a monocrystal, such as sapphire. Thebase bodies 23, 25 are each connected to the membrane 27 by a respectivejoint 32, 34, for instance by means of an active hard solder, such as azirconium-iron-titanium-beryllium solder, each at a respective outerannular edge.

In operation, a first pressure p1 prevails in the measuring chamber 29,and a second pressure p2 prevails in the further measuring chamber 31.The deflection of the membrane 27 depends on a pressure to be measured,which is equivalent to the difference between the first and secondpressures p1 and p2.

For delivering the pressure, the base body 23 has a continuous bore 37,leading into the measuring chamber 29, and into which a pressure tube 39is introduced. In operation, the first pressure p1 is delivered to themeasuring chamber 29 via the pressure tube 39. Analogously, for pressuredelivery, the further base body 25 has a continuous bore 41, which leadsinto the measuring chamber 31 and into which a pressure tube 43 isintroduced. The second pressure p2 is delivered to the measuring chamber31 in operation via the pressure tube 43.

In the exemplary embodiment shown, each of the pressure tubes 39, 43communicates with a respective pressure mediator 45, 47. Each pressuremediator 45, 47 has a respective partitioning membrane 49, 51, each ofwhich covers a respective chamber 53, 55. From outside, the firstpressure p1 acts on the partitioning membrane 49, and the secondpressure p2 acts on the second partitioning membrane 51. The chambers53, 55, the pressure tubes 39, 43, and the measuring chambers 29, 31 arefilled with a substantially incompressible fluid, such as a siliconeoil. By means of the fluid, the first and second pressures p1, p2,acting on the partitioning membranes 49, 51 from outside, are eachtransmitted to a respective side of the membrane 27.

Precisely as with the pressure measuring cells described above, onceagain capacitors are used as electromechanical transducers. In thispressure measuring cell as well, an electrode 33 is applied to amembrane bed, formed by a surface toward a membrane of the base body 23,and this electrode, together with a counter-electrode 35 applied to themembrane 27, forms a capacitor whose capacitance is a measure for thedeflection of the membrane 27. Via the joint 32 adjacent to it, thecounter-electrode 35 is connected electrically conductively to areference potential, such as ground.

The left half of the differential-pressure measuring cell of FIG. 3having the base body 23, the membrane 27, the electrode 33, and thecounter-electrode 35, is in principle similar to the measuring cellsshown in FIGS. 1 and 2. However, as already noted, the measuring chamber29 is fluid-filled and is connected to the pressure mediator 45.

Just as in the preceding exemplary embodiments, once again here acontact pin 59 is provided, which is passed through a bore 57 throughthe base body 23 and by way of which the electrode 33 for measuring thecapacitance is electrically connected. The contact pin 59 is soldered inplace on the side toward the membrane bed in the bore 57 with a solder61, and the contact pin 59, solder 61, and membrane bed form a smooth,for instance polished, surface.

Preferably, the right half of the differential-pressure measuring cellis constructed identically to the left half and has an electrode 67,applied to a surface oriented toward the membrane of the base body 25,which electrode together with a counter-electrode 69 applied to themembrane 27 forms a capacitor, whose capacitance is a measure for thedeflection of the membrane 27. Via the joint 34 adjoining it, thecounter-electrode 69 is electrically conductively connected to areference potential, such as ground. Just as on the left-hand side, onthe right-hand side a contact pin 73 passed through a bore 71 throughthe base body 25 is also provided, by way of which the electrode 67 formeasuring the capacitance is connected electrically. The contact pin 73is soldered into the bore 71 with a solder 75 on the side toward themembrane bed, and the contact pin 73, solder 75, and the membrane bedform a smooth, for instance polished, surface.

Besides the advantages already named, the smooth surface in theexemplary embodiment shown in FIG. 3 has the additional advantage thatthere are no recesses that could be penetrated by an undefined quantityof fluid. A precisely constant volume of fluid which is as small aspossible and is as much as possible identical in both halves is animportant prerequisite for achieving high measurement precision.

Also analogously to the exemplary embodiments described previouslyabove, the contact pins 59, 73 can also be soldered on a side remotefrom the membrane bed into the base body 23 by means of a solder 62 andinto the base body 25 by means of a solder 77. However, in mostapplications, soldering them onto only a single side toward the membranebed will suffice.

The contact pin 59 is connected to an electronic circuit 63, whichdetects the instantaneous capacitance of the capacitor and converts itinto an electrical output signal, which is available for furtherprocessing and/or evaluation via connection lines 67.

Analogously, the contact pin 73 is connected to an electronic circuit79, which detects the instantaneous capacitance of the capacitor andconverts it into an electrical output signal that is available forfurther processing and/or evaluation via connection lines 81.

Preferably, a difference between the two capacitances is formed, andfrom that, the pressure difference acting on the pressure measuring cellis ascertained.

The production method for the differential-pressure measuring cell shownin FIG. 3 is essentially equivalent to the production method describedearlier above. Accordingly, only the differences will be described infurther detail below.

These differences essentially comprise the fact that the membrane 27 isconnected to the base body 23 and to the further base body 25 by meansof a joint. This can be done in a single operation. In this operation,preferably the first pressure tube 39 is also soldered into the basebody 23 and the second pressure tube 43 is also soldered into thefurther base body 25.

The pressure tubes 39, 43 for instance comprise a special steel and aresoldered into the base bodies 23, 25 for instance with an active hardsolder, such as a silver-copper solder. Alternatively, higher-qualitymaterials can be used for the pressure tubes 39, 43. Examples aretantalum or iron-nickel-cobalt alloys, of the kind attainable under thetradenames Kovar or Vacon.

Solder is first placed between the membrane 27 and the first and secondbase bodies 23, 25 and is introduced into the bores 37, 41. Next, thepressure tubes 39, 43 are introduced, and the measuring cell is placedin a furnace, in which the soldering operations are is done in a vacuumor in a protective gas atmosphere.

Naturally, the invention is not limited to pressure measuring cellshaving a single electrode. In all the exemplary embodiments described,instead of the single electrodes 7, 31, 67, it is also possible for twoor more electrodes to be used for picking up a measured value and forinstance for calibration purposes as well. In that case, the electrodesare disposed on the respective base body 1, 23, 25, and the individualelectrodes are each connected in the manner described above by means ofcontact pins soldered into suitable bores.

1. A pressure measuring cell, having: a base body; a membrane, connectedto said base body, forming a measuring chamber, which in operationexecutes a deflection that is dependent on a pressure to be measured; amembrane bed, to which said membrane conforms in the event of anoverload, said membrane bed being formed by a surface, oriented towardsaid membrane, of said base body, onto which bed at least one electrodeis applied, which together with a counter-electrode applied to saidmembrane forms a capacitor, whose capacitance is a measure for thedeflection of said membrane; and a contact pin, passed through a borethrough said base body, by way of which pin said at least one electrodeis electrically connected for measuring the capacitance, and whichtoward said membrane bed is soldered into said bore with a solderwherein: said contact pin, said solder, and said membrane bed form asmooth surface.
 2. The pressure measuring cell of claim 1, wherein: saidbase body and/or said membrane is of ceramic or a mono crystal.
 3. Thepressure measuring cell of claim 1, wherein: said contact pin comprisestantalum, and said solder is an active hard solder comprisingsilver-copper solder.
 4. The pressure measuring cell of claim 1,wherein: in operation a pressure to be measured acts on said membrane;and a very small pressure near 0 mbar or a reference pressure that isdelivered through said base body prevails in said measuring chamber. 5.The pressure measuring cell of claim 1, wherein: said membrane isdisposed between said base body and a further base body; said membraneand said further base body form a further measuring chamber; inoperation, a first pressure prevails in said measuring chamber; inoperation, a second pressure prevails in said further measuring chamber;and the deflection of said membrane depends on a pressure to bemeasured, which is equivalent to the difference between said first andsaid second pressure.
 6. The pressure measuring cell of claim 5,wherein: said base body has a continuous bore leading into saidmeasuring chamber, into which bore a pressure tube is introduced, by wayof which in operation said first pressure is delivered to said measuringchamber; and said further base body has a continuous bore leading intosaid measuring chamber, into which bore a pressure tube is introduced,by way of which in operation said second pressure is delivered to saidmeasuring chamber.
 7. A method for producing a pressure measuring cell,the pressure cell having: a base body having a bore; a contact pin; amembrane bed; an electrode; and a first pressure tube, the methodcomprising the steps of: inserting a contact pin into a bore;introducing solder between the base body and the contact pin toward themembrane bed; soldering the contact pin in; polishing the membrane bed;applying the electrode to the smooth membrane bed; and connecting thebase body and the membrane solidly together by joining.
 8. The methodfor producing a pressure measuring cell of claim 7, wherein: themembrane is connected to the base body and to the further base body bymeans of a joint; and the first pressure tube is soldered into the basebody and the second pressure tube is soldered into the further basebody.